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Printed Electronic Circuit Process for LED Interconnection
Thursday, January 07, 2010 | Mike DuBois, Caledon Controls Ltd., Printed Electronics World

Higher wattage LED's/power components or their placement in higher densities, requires a larger dissipation of heat in a more effective way.

The following is quoted from Canadian Electronics magazine by Gijs Werner, FCI, in December, 2009:

"With rapid advancements in power reduction, and minimizing and controlling LED heat, it is expected that future applications will use less-expensive, rigid aluminum-backed printed circuit boards for proper heat dissipation and move towards more cost-effective flexible circuits. This is also being driven by signal and display applications, where most of the future LED growth is expected."

PCB mounting of LEDs has, to date, been limited to mechanical interconnection or the use of PCBs built on thermally conductive copper clad aluminum substrates.

These laminated substrates, which are available from several specialty laminate manufacturers, typically consist of a copper foil, which is laminated to the aluminum using a glass cloth impregnated with heat conducting additive filled epoxies. The cloth offers small window openings between the weave patterns where the filled epoxies can make contact with both the copper foil and the aluminum to transfer heat. The glass cloth which comprises 50% of the prepreg, significantly reduces the thermal conductivity. Additionally the existing available thermal prepregs are thicker at 100 microns (0.004 mils) which increases the effective thermal resistance limiting the maximum watts per square inch dissipation.

A PCB manufacturer would employ this laminate material, laminating photo-imageable etch resist, exposing with UV light then developing to form a circuit image, then etch away unwanted copper to produce circuit traces. All exposed aluminum must also be well masked off, a time- and material-consuming task.

Printed Electronic Inks

Enhanced thermally conductive inks are much more versatile. They can be used to make a more efficient thermally conductive copper clad aluminum laminate substrate by the PCB fabricator themselves (Figure 1) or could be used by selectively printing both the dielectric and conductive traces to make a thermally conductive circuit by anyone with a silk screen printer and a thermal oven (Figure 2).

Using CCD-120A thermally conductive dielectric ink with its nanotechnology-enhanced formulation in making PCB boards or even making copper clad aluminum circuit laminate, will deliver lower thermal resistance than that of other currently available copper clad aluminum substrates.

Copper Clad Aluminum

The thermally conductive B stage ink is applied to either the surface of the copper foil or of the aluminum and is semi cured (B-Stage) using a thermal oven. Application is typically a silk screening operation to deposit a thin film of thermal ink approximately 0.001" thick. The ink is then baked at 250°F for 20 to 30 minutes, at which time the material reaches the desired B stage cure.

The coated B Stage aluminum is laminated with the copper foil of choice using a typical vacuum assist laminating press and fully cured under pressure.

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